Comment The presence of HIV in semen has been well documented,2 3 but the relation between the viral load in semen and peripheral blood CD4 counts is not a simple one.' No studies have looked at the serial viral load of genital fluids during treatment for other sexually transmitted diseases, although a single case report has suggested that chlamydial urethritis may increase shedding of HIV-1 in the semen.4 Our results help explain how transmission of HIV may be facilitated by concomitant sexually transmitted diseases and add further support for an aggressive approach to treating sexually transmitted diseases in HIV infected patients, as a means of reducing transmission of HIV and for reinforcing the benefits of using condoms.5
Use of the capture-recapture technique to evaluate the completeness of systematic literature searches Pat Spoor, Mark Airey, Cathy Bennett, Julie Greensill, Rhys Williams Capture-recapture methods were pioneered in ecology and derive their name from censuses of wildlife in which several animals are captured, marked, released, and subject to recapture. In epidemiology the technique examines the degree of overlap between two (or more) methods of ascertainment and uses a simple formula to estimate the total size of the population. When the number already identified is subtracted from this estimate the number of cases not ascertained by either (or any) of the methods can then be calculated. It has been suggested that studies which attempt to ascertain all cases of a given disease in a population should use this method to estimate the number of missing cases."2 There are direct parallels between epidemiological studies which attempt to ascertain all available cases and systematic literature searches which attempt to identify all publications on a given topic: both should incorporate estimates of the number of cases or publications they fail to identify. Our study compared, for one journal, the results of searching an electronic literature database with those of hand searching, both carried out for the Cochrane collaborative review group on diabetes.'
Division of Public Health, Nuffleld Institute for Health, University of Leeds, Leeds LS2 9PL Pat Spoor, information manager Mark Airey, senior research fellow Cathy Bennett, research fellow Julie Greensill, research assistant Rhys Williams, professor of epidemiology and public health Correspondence to: Pat Spoor.
[email protected] BMJ 1996;313:342-3
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Methods and results The Medline database was searched from January 1984 to October 1994 for articles in Diabetic Medicine likely to be describing randomised controlled trials, as defined by Dickersin et al and using their search strategy.4 Independently, a handsearch of the journal for the same period was carried out, with the same aim. The maximum likelihood estimator, N = M(n/m), was used to estimate the total population size,2 where M is the number of publications identified by Medline, n the number identified by hand searching, and m the number identified by both sources. The estimated number unidentified by either method was calculated by subtraction. The maximum likelihood estimator is biased for small samples, for which Chapman's method is more appropriate.5 This estimates the total population size as N = (M + 1)(n + 1)/(m + 1) - 1. The variance of N is estimated as Var(N) = (M + 1) (n + 1) (M - m)(n - m)/((m + 1)2(m + 2)), from which 95% confidence intervals can be constructed.
Funding: Part of this work was supported by the UK Medical Research Council and by the St Stephens AIDS Trust. Conflict of interest: None. 1 Gaynor R. Cellular transcription factors involved in the regulation of HIV- I gene expression. AIDS 1992; 6:347-63. 2 Zagury D, Bernard J, Leibowitch J, Groopman JE, Feldman M, Gallo RC. HTLV-III in cells cultured from semen of two patients with AIDS. Science 1984; 226:449-5 1. 3 Vernazza PL, Eron JJ, Cohen MS, van der Horst CM, Troiani L, Fiscus SA. Detection and biological characterization of infectious HIV- 1 in semen of seropositive men. AIDS 1994;8: 1325-9. 4 Eron J, Gilliam B, Fiscus S, Dyer J, Cohen MS. HIV-1 shedding and chlamydia urethritis. AAMA 1996;275:38. 5 Grosskurth H, Mosha F, Todd E, Murijarubi E, Klokke A, Senkoro K, et al. Impact of improved treatment of sexually-transmitted diseases on HIV-1 infection in rural Tanzania: randomized controlled trial. Lancet 1 995;346:530-6.
(Accepted 24 April 1996)
Table 1-Extent of overlap in the number of publications found by Medline search of "Diabetic Medicine" (January 1984 to October 1994) and by hand searching Medline search
Hand search
Hand earch
Found
Not found
115 8
35
Found Not found
2'
*Estimated by capture-recapture technique, rounded to the nearest whole number.
Table 1 shows the number of publications identified by each method and the overlap. The articles missed by the hand search are attributed to human error; those not identified by the Medline search were improperly indexed, either because until recently no appropriate methodological subject heading existed or because the abstract failed to describe the study design. For our data both the maximum likelihood estimator and Chapman's method gave the same estimate of total population size (160, 95% confidence interval 158 to 164) rounded to the nearest whole number. The number of articles "missed" was 2 (0 to 6).
Comment A caveat to the application of these methods is that if there is positive dependency between the two sourcesthat is, if an article identified by hand searching is more likely to be ascertained in Medline than one not so identified-then the estimates will underestimate the true population. If, however, Medline and the hand search are negatively dependent then the estimates will overestimate the true population.2 Log-linear modelling offers an alternative approach to modelling dependency among data, where it is present. The term capture-recapture is not so appropriate for the technique's use in epidemiology or literature searches since, while cases and publications may be said to be "captured," nothing is being "recaptured." As applied in epidemiology the method has been termed "ascertainment intersection."2 However, we suggest the more informative descriptor "comparison of multiple methods of ascertainment" (or COMMA) for this useful technique, which we advocate for all systematic literature searches. We thank Mr Alan S Rigby for statistical advice. Funding: Northern and Yorkshire Regional Health Authority and the British Diabetic Association. Conflict of interest: None.
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10 AUGUST 1996
1 LaPorte R. Assessing the human condition: capturerecapture techniques. BM_ 1994;308:5-6. 2 Hook EB, Regal RR. The value of capture-recapture methods even for apparent exhaustive surveys. The need for adjustment for sources of ascertainment intersection in attempted complete prevalence studies. Am J Epidemiol 1992;135:1060-7. 3 Airey CM, Williams DRR. Cochrane Collaborative Review Group: Diabetes. Diaber Med 1995;12:375-6.
Iron deficiency anaemia and febrile convulsions: case-control study in children under 2 years Alfredo Pisacane, Renato Sansone, Nicola Impagliazzo, Angelo, Coppola, Paolo Rolando, Alfonso D'Apuzzo, Ciro Tregrossi Febrile convulsions are the most common type of seizure and occur in 2-4% of all children.' A family history of convulsions; maternal smoking; and alcohol consumption during pregnancy have been associated with febrile seizures,2 3 but the risk factors remain largely unknown. Iron is involved in the metabolism of several neurotransmitters, and monoamine and aldehyde oxidase are reduced in iron deficiency anaemia4, which is common during the second and the third year of life and has been associated with behavioural and development disturbances.' Thus we investigated the association between iron deficiency anaemia and febrile seizures by a case-control study.
Dipartimento di Pediatria, UniversitA di Napoli, Via Pansini 5, 80131 Naples, Italy Afredo Pisacane, senior
lecturer Renato Sansone, postgraduate trainee Nicola Impagliazzo,
postgraduate trainee Angelo Coppola, postgraduate trainee Divisione di Pediatria, Ospedale di Castelanmmare di Stabia, Naples Paolo Rolando, paediatric consultant Alfonso D'Apuzzo, paediatrician Ciro Tregrossi, paediatrician
Correspondence to: Dr Pisacane.
Patients, methods, and results All 156 children aged 6-24 months admitted to Castellammare di Stabia Hospital, Naples, between 1 January 1993 and 30 June 1995 with diagnosis of febrile convulsions were enrolled in the study. They were healthy children without previous afebrile seizures or central nervous system disease. A febrile convulsion was defined as a seizure that occurred while the child had a rectal temperature of at least 38.3°C or an axillary temperature of at least 37.8°C documented either in the emergency department or in the history. Tvo groups of controls were selected: a random sample of children admitted to the same ward with diagnosis of respiratory and gastrointestinal infection during that period, and a group of healthy children randomly selected from the provincial birth register for an iron deficiency survey in Greater Naples during 1994. Routine haematological investigations were performed for hospitalised patients at hospital admission and for population controls at the Department of Paediatrics of Naples. Data were collected from clinical records by two medical students unaware of the study hypothesis.
4 Dickersin K, Scherer R, Lefebvre C. Identifying relevant studies for systematic reviews. BMJ 1994;309:1286-91. 5 Chapman DG. Some properties of the hypergeometric distribution with applications to zoological sample census. University of California Pubic Stausscs 1951;1:131-60.
(Accepted 6 March 1996)
Iron deficiency anaemia was defined as the presence of haemoglobin concentration
